Maintenance and cleaning of plasticising cylinder

 Clean when it is absolutely necessary to clean

 If it is desired to change from one plastic to another, it is advisable to observe the correct sequence for colour change or material change. This saves a great deal of cleaning work.

 A decision whether to clean partially or totally depend on the type of contamination.

 In most cases, contamination due to material entrapment results if there is a gap between plane sealing surfaces of the injection unit – especially near the surfaces of nozzle and cylinder head or near the cylinder head and barrel.

Partial cleaning

It is frequently found that the screw tip and the conical surface of the bore in the cylinder head are the areas where remnants of material get entrapped and adhere. Streaks which occur regularly at the same point in the moldings are most likely due to such entrapped material. If the remainder of the melt is in other aspects completely free from defects, it is sufficient to clean the sealing surfaces, the bore and the sealing faces of the head and to check the screw tip.

Total cleaning

If the contamination of the mold is irregular and finely distributed, total cleaning is necessary.

If discoloration, dark areas or even charred areas are found at the remotest point of the flow path or in the region of the mold which is last to be filled, then these could be due to air entrapment. This in turn could be due to incorrect mold design, incorrect gating or inadequate mold venting and are not due to contamination of screw cylinder.

Preparatory activities

 If the screw cylinder is to be cleaned, it is essential to purge it empty after closing the hopper shut off slide.

 Retract the cylinder after purging

 If materials which are more difficult clean are processed, e.g., PA, PC and the like, it is essential after purging to plasticise some acrylic or PS or PE and again squirt the cylinder empty and retract the screw.

Note :

Before changing the material, check the temperature of the screw cylinder

Removing the screw cylinder

 Before removing the screw cylinder assembly, make sure that it is empty.

 By actuating the manual operation key, retract the injection unit fully to its end position.

 Untie the electrical connection bunch (heater cables and thermocouples).

 Switch off the heaters and remove all thermocouples;

Unplug heaters

 Bunch heater cables and thermocouple cables                    separately. 

 Remove feed throat water connections after closing the

    necessary shut off valve for feed throat cooling.

 Keep the hopper shut off slide closed, pivot the hopper      out of the way and remove it.

 Retract the screw slightly to gain access to the flange    

 Switch off the pump.

 Switch on the pump and retract the screw till the spline      s of the screw shaft becomes disengaged. Switch off          the pump.

 Hang the screw cylinder on a crane at the eye bolts e.

DO NOT LIFT YET!

Attention!

As a result of the distribution of the heater bands and various other cylinder fitments, the eye bolts are not positioned at a balancing point. In order to prevent accidents, this must be kept in mind while lifting.

 Loosen the grub screw g. Loosen the nut z by one or          two turns.

 Dismantle the block b by removing the six bolts on top      of the block.

 Now the screw cylinder has become free and can be          lifted out of the carriage horizontally.

 Place the screw barrel assembly close to the machine        control panel at a comfortable working height.

Cleaning the screw cylinder

The cleaning of the screw cylinder should be carried out by an experienced personnel. While cleaning the cylinder, foul smelling inflammable gases are at times released and these can be a nuisance on the shop floor. Keep the area well ventilated.

 Place the thermocouples inside their respective housings.        Connect back the heater plugs into their respective sockets.

 Switch on the heaters. Allow the temperatures to rise to the      upper limit allowable for the material.

 Continue heating until the screw can be freely rotated. Bear      in mind the time explosion hazards presented by various          plastics. Switch off the heaters.

 Unscrew and remove the nozzle.

 Take the heater band off the head and unplug it. Unscrew        the head.

 Switch on the heaters. Draw out the screw in the forward          direction (towards the nozzle side) repeatedly upto the end      of screw profile and allow the melt to drip off.

 Draw out the screw by pulling it out from the spline end.            Allow the material to drip off while hot.

 Continue heating the material till the residues of the                  material have melted off the cylinder wall.

 If necessary, wrap rags around a wooden stick and use this      to remove additional remnants of plastic sticking to the              cylinder wall.

 Clean the threads on the cylinder with a wire brush.

 If the cylinder is to be put into storage, it should be sprayed     with a rust-proofing agent. Before re-assembly, however,         degrease and treat all threads with anti-seize compound.

Cleaning the screw

 The melt which remains stuck to the screw even after        dripping off and simple removal process should be            burnt off with the weakest possible flame. A propane          gas burner is recommended.

 Do not concentrate flame on small areas. Heat large            areas uniformly and, at the same time, turn the screw        slowly. Unbalanced heating causes screw distortion.

 The surface temperature of the screw must not exceed      400 O C, otherwise hardness and wear resistance may       be adversely affected.

 Caution is needed when heating with the non-return            screw tip screwed on. The thin walled locking ring              heats up much rapidly.

 Finally clean the screw with steel wire brush. Polish the      screw with emery cloth, 240-280 grade.

 If the material has remained in the internal thread (left       handed thread of the screw) clean the thread                       thoroughly  with a steel wire brush.

 While screwing in the tip, use molybdenum disulfide          based grease for lubricating the thread.

Cleaning the screw tip

Removing the screw tip

 Clamp the screw tip b horizontally into the vise with its using the V – shaped aluminum jaws m.

 Do not damage the check ring or non return valves.

 Support screw c on shaft f.

 Insert screw coupling e into the spline shaft key

Attention!

Bear in mind that the thread is left handed

 Rotate the spline shaft key to loosen the tip.

 After having detached the screw tip, locking ring and thrust ring from the screw, give them a final clean with the steel brush and polish them with emery cloth 240-280 grade.

 Clean the threads of the screw tip thoroughly with steel brush and apply molybdenum disulfide based grease.

Cleaning the nozzle

 With flame, burn off the material still clinging.

 Clean the orifice and make sure that the orifice is not blocked

 Clean with a wire brush and polish with emery cloth 240-280 grade

Attention!

Do not round off the edges of the plane sealing surfaces

 While screwing back, apply molybdenum based grease on the threads.

Checking and adjusting nozzle centering

 Remove the mould from the machine.

 Heat up the purged cylinder with screw in forward position to 200C.

 Maintain this temperature for all zones for one hour.

 Bring the injection unit forward so that the nozzle tip is flush with the surface of the fixed platen.

 Center the nozzle as per the following procedure:

1. Loosen the bolts of the injection unit

2. Correct the vertical deviation by adjusting the injection unit lifting screw.

3. Adjust horizontal deviation by tapping injection unit from sides.

4. After adjusting the centricity, check for canting of the injection unit when fully retracted.

5. If necessary correct canting by adjusting the lengths of piston rods.